Terrestrial animals use binaural processing to help them detect, discriminate, locate, and segregate sounds sources. All regions of the central auditory system participate in the computations used for these general tasks of hearing, but several nuclei of the mammalian and avian brainstem have been most intensively studied with respect to binaural processing. Recent observations on fishes, including behavior, central auditory system morphology and peripheral and central neurophysiology converge in suggesting not only that the flow of information from the periphery to the epencephalon follows the same general pattern in all classes of jawed vertebrates, but that the sense of hearing itself may be homologous within these taxa, having first developed among early fishes. This, in turn, suggests that binaural processing may have first developed in early fishes, too. In this proposal, we aim to systematically investigate, for the first time, the strategies and mechanisms of binaural hearing in a fish (oyster toadfish - Opsanus tau). These studies using combinations of natural acoustic and bilateral mechanical otolith manipulation, in vivo, will provide the first systematic description and analysis of binaural processing in a fish, and will help determine what binaural mechanisms observed in terrestrial animals are primitive, or shared among vertebrates, and what mechanisms are likely to have been more recently derived. We hypothesize that several fundamental structural and functional aspects of binaural processing are primitive vertebrate characters and we expect them to be primitively revealed in the auditory brainstem of fishes. This work will contribute to our understanding of the evolution of auditory systems and the sense of hearing, and thereby help establish a biological and evolutionary context within which human hearing and animal models for human hearing can be better understood and applied.